1. Field of the Invention
The present invention relates to injection molding machines, and, more particularly, to the die closing unit of an injection molding machine and to a device for lubricating the tie rods of such a die closing unit.
2. Description of the Prior Art
The prior art in the field of injection molding machines encompasses a number of different machine types, with an assortment of different die closing units. These die closing units, with few exceptions, have in common that they use two or four parallel tie rods to provide the guidance for the movable die half and its supporting structure during the die closing and opening movements. The accuracy and rigidity of this guidance is of great importance, both for a consistant quality of the machine output and for the longevity of the die closing unit. An important factor in maintaining this accuracy and longevity is a reliable supply of lubricant for the tie rods and the cooperating slide bushings.
It is well known to arrange for a supply of lubricant to a reciprocating guide assembly by having an annular lubricant holding chamber arranged inside a sleeve, between axially spaced guide bushings. Such an arrangement is shown, for example, in U.S. Pat. No. 4,094,621 which discloses a die closing unit for an injection molding machine featuring a heavy movable die carrier frame which is guided for opening and closing movements on four tie rods. The die carrier frame, consisting of axially spaced transverse walls which are connected by four hollow guide sockets and reinforcing ribs, has short guide bushings arranged in both ends of each guide socket. The intermediate length portions of the guide sockets thus form chambers which, when periodically filled with lubricant, provide a good lubricant supply.
Another tie rod lubrication system is disclosed in U.S. Pat. No. 3,833,204, where two tie rods which carry an injection unit are lubricated with hydraulic fluid. Tubular piston rods, surrounding a length portion of the two tie rods, form annular lubricant holding chambers into which lubricant is continuously fed from the pressure spaces of the associated injection cylinders, via helical lubricating grooves arranged in the tubular piston rods.
The first-mentioned lubrication system has the shortcoming of being limited to a grease lubricant, with its disadvantage of not freely flowing to the place where it is consumed. It therefore requires frequent refills under pressure. The second prior art solution is limited to tie rods which support a tubular piston rod, or tie rods which double as piston rods, where a hydraulic pressure space providing a ready supply of hydraulic fluid is available.